It sounds like you have a vacuum problem. When the engine is running it generates a vacuum in the exhaust manifold. That vacuum is used to move the actuators inside your dash to the position you chose with your controls. If you have a vacuum leak or something interrupts the vacuum, the actuator will move to its default position which is the defrost vents. It sounds like the act of accelerating is somehow interrupting the vacuum. I'd look for little cracks in your vacuum lines and check your ERG valve.

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There is a flapper or trap door in the dash board that diverts air flow to various vents, defrost, floor, face, etc. If that stops working it will get stuck in the default position which is usually the defrost vents.

If you have a vacuum operated system you can check the vacuum hose for the system in the engine compartment for integrity. If that is ok then you have to get to the vacuum switch in the dashboard and probably replace it due to leaking vacuum - labor intensive and not cheap.

Any time the dashboard has to be removed you are in trouble money wise.

I just fixed mine. The lever that opens and closes a door that directs the air flow to the defroster was broken. To be more specific, the plastic lever the actuator attaches to was broken. The actuator and the vacuum system were operating properly. It is simple to see but hard to get to. Remove the glove box and look up and to the left for a silver vacuum controlled actuator. I was able to fabricate another lever and attach it to what was left of the OEM lever.

sounds like your fan is either jamned, burnt out or even frozen, you might be lucky and find its just a blown fuse, you'll need to check your fuses box then start investigating if the fan turns freely or not

if you put the air on recirc you might be able to stop cold air blowing in when you are travelling. if you are waiting for a garage to sort out the fault then get a fan powered by the cigarette lighter, air moving over the window should at least keep mist clear even if it doesnt defrost it

I had a similar issue on an 05 silverado. Air would only blow out of defrost vents. But this worked for me:

1) Unplug HVAC fuse for 10-30 seconds
2) Start truck
3) A/C comes out defrost - wait about 10 to 15 secs
4) A/C then starts coming out the floor vents for about 5 seconds
5) A/C then starts coming out the dash vents.
6) While the A/C is coming out the dash vents, turn A/C OFF (otherwise it cycles back to defrost for good).
7) Turn truck OFF
8) Restart truck and it should work fine